Ever since man first endeavored to build and operate heating plants within inhabited structures for the heating thereof or for otherwise providing a useful energy source, two related problems have been almost universally recognized. One is that heating plants, which operate by the controlled combustion of fuel-oxygen mixtures, can liberate substantial amounts of toxic, poisonous or explosive vapors as well as purge the structure of breathable oxygen. The second problem arises from attempts to remedy the first, that is, from the venting of the vapor's products of combustion from the structure by use of chimneys, stacks, flues or the like. The combustion taking place in the heating plant, being exothermic in nature, heats the vapors liberated thereby. If the vapors are vented from the structure, so to is a significant quantity of heat, thereby reducing the amount of heat remaining to warm the structure or perform other useful work. Restated, substantial inefficiencies result from the venting process.
When fuel was inexpensive and plentiful, gross inefficiencies in the heating plants were accepted. However, recent awareness of diminishing reserves of fuel and astronomical increases in fuel prices, particularly in hydrocarbon based fuels, have accentuated the need to improve heating plant efficiency. Furthermore, the long term need to reduce total heat emissions on a global basis to avoid a "greenhouse effect" is being postulated by an increasing segment of the world scientific community.
The basic problem of improving heating plant efficiency is that of somehow preventing the escape of flue gas heat while, at the same time, insuring safe and complete venting of the vapors. Two basic approaches are found in the prior art. One approach is the use of thermally responsive dampers which close or restrict flow through the exhaust flue during certain portions of the heating plant's operating duty cycle. Such devices are only partially effective and can present a substantial safety risk in some failure modes. Additionally, such devices typically only operate during off or rest modes and have a de minimis effect on efficiency during "burner on" operation of the heating plant.
The second approach of recovering waste heat in an exhaust flue found in the prior art is embodied in apparatus employing secondary heat flow paths to recycle or regenerate waste heat into useable heat. Such approaches typically include a heat exchanger series connected with the exhaust flue to absorb thermal energy from the heated flue gases passing therethrough. The heat exchanger has a liquid circulating therethrough in a closed loop with a second heat exchanger located in the cold air return. The liquid carries the absorbed thermal energy to the second heat exchanger which, in turn, transfers it to the air flowing in the cold air return. The net effect of this transfer is to reduce the inlet-outlet temperature differential of the heating plant. With the reduction in differential, the plant requires less fuel to maintain the associated heated structure at the same level.
A typical prior art device, such as that described immediately herein above, is disclosed in U.S. Pat. No. 4,136,731 to DeBoer, which discloses a heat transfer system for use in supplementing the operation of a heating/cooling system of a building and its hot water heating system. The device includes a heat exchanger in the flue of the furnace as well as a heat exchanger in the fan chamber. A first liquid circulation loop couples the heat exchangers for transferring heat from the flue exchanger to the air moved through the fan chamber heat exchanger. A second liquid circulation loop includes a flue exchanger and the building hot water heater for supplementing the heating of water therein. In the summer months, during the cooling mode of the system's operation, cold water employed, for example, for lawn sprinkling is passed through the fan chamber heat exchanger for cooling and dehumidifying air circulated in the building. A valve control system is employed to automatically control the flow path of fluid in the system as a function of detected temperatures.
Alternative prior art approaches employ air as the intermediate media for the capturing of flue gas heat. Examples of such an approach are disclosed in U.S. Pat. No. 3,944,136 to Huie and U.S. Pat. No. 4,163,441 to Chen. Huie discloses a heater unit and draft control adapted to be connected to an existing solid, liquid or gas fuel heating furnace which includes a heat exchanger having a stack gas passage connected in series to a stack from the furnace and a blower which forces air through the heat exchanger and into the plenum of the furnace. A stack cooler and/or draft control includes a pipe extending from outside the heated structure into the exit portion of the stack gas passage with an adjustable damper therein. Chen discloses an air duct disposed between the top end of the heat exchanger of a furnace and the entry port into the flue pipe of the furnace. An electrical blower is coupled to the duct to draw hot air therethrough which would otherwise go out to the atmosphere through the flue. The hot air drawn by the blower is redirected to heat, or assist in heating, the building to which the furnace is directing heat or heat some other facility. An electrical control circuit is connected to the blower and controls the blower such that if the furnace is burning fuel the blower cannot be turned on. The blower can only be turned on after a predetermined time has elapsed from the time that the fuel burning has terminated, so that combustion gases which otherwise would pass through the flue and which might contain harmful ingredients therein, will not be redirected to be used as a source of heat.
The ever increasing cost of most fuels, as in the last decade, caused developers of heating plants to seek further efficiency improvements which heretofore were not cost effective. One approach was to remove even more heat from flue gases, and to remove heat from flue gases during a greater portion, or all, of the heating plant duty cycle. In squeezing out the last few points of efficiency however, new concerns have arisen, particularly those of providing adequate ventilation of the fumes of combustion and insuring that those fumes are not intermixed with the fresh heated media being circulated within the inhabited structure.
A basic problem arisig from removing heat from gases is that it reduces the tendency to naturally asperate to the atmosphere outside of the heated structure by "chimney effect". This tendency exacerbates the fume leakage problem within the structure as does the recycling of flue gases within the heating plant during periods of operation when the heating media is being circulated within the structure.
A relatively recent prior art approach to these problems was to provide a heat exchanger within the cold air return and a power vent such as a small fan, which draws some or all of the flue gases through the heat exchanger to heat the air therein and then discharge the cooled flue gases externally of the heated structure. This arrangement provides efficient removal of much of the heat from the flue gases. A somewhat serendipitous advantage of this arrangement is that it allows horizontal venting of at least some of the flue gases through the side of the structure, eliminating the need for a vertically rising stack extending through the structure's roof level.
Although a clear improvement over many prior art approaches, the arrangement described immediately herein above does have some disadvantages. The flue gases being vented are still relatively hot as they exit the structure, representing a safety hazard as well as lost heat which could potentially be turned to useful heat. Also, the additional complexity of such systems increase cost and potential failure modes.
It will be apparent from a reading of the specification that the present invention may be advantageously utilized with many different heating plants which circulate a media heated by combustion of a fuel-air mixture which requires the venting of flue gases therefrom, such as steam powered generating plants, refineries, foundaries, factories and many other commercial and residential applications. However, the invention is especially useful when applied to domestic forced air or hydronic furnaces and will be described in connection therewith.